Zimbra allows relaying emails to a specific server only, i.e using zimbraMtaRelayHost. However, there are needs to relay emails per domain basis based on the sender. For example, you want emails sent by users@domain1.com should be relayed through smtp.domain1.com and users@domain2.com through smtp.domain2.com servers. You can configure it using postfix's "sender_dependent_relayhost_maps".

This is an example that has been tested on ZCS 5.0.16 and ZCS 7.1.4

Note: Below settings will not survive zimbra upgrades. Make sure you take backup of config files before upgrading.